Tuesday, October 18, 2011

Death of Armed Police in Nepal due to Blast of Computer

CRT Monitor Health Hazard
i am quite socked seeing the news about the blasting of computer leading to death of police officer at Birgunj, Nepal. especially CRT (Cathode Ray Tube) monitor operates at high voltage and is the energy reservoir. CRT system retains the high voltage energy for a long period of time even if it is shutdown. May be the following literature helps find out the cause of computer blast in Birgunj.

CRT Monitor Health Hazard
Have you ever walked across a carpeted floor and gotten a shock when you touched a doorknob, table, counter, or even another person? that little shock you got was a result of static electricity. It was also many times what is needed to destroy some computer components.
You see, humans can't feel a static shock until it is several thousand volts strong, but it takes less than 30 volts to damage a sensitive computer component, such as a stick of RAM or a processor.
That's why computer technicians and home computer builders have to guard their computers against the deadly ravages of static electricity, as well as take steps to avoid injury to yourself. A shock that you can't even feel can seriously damage your homebuilt computer before you're even finished building it.
Electromagnetics: Some believe the electromagnetic fields emitted by CRT monitors constitute a health danger to the functioning of living cells. Exposure to these fields diminishes by the inverse square law which describes the propagation of all electromagnetic radiation: double the distance, quarter the power. Likewise, the EM energy is also less intense for the display's user than for a person located behind it because the deflection yoke is behind the display's screen and therefore closer to the rear. It is well-known that electromagnetic waves of sufficient intensity can harm human cells (see ionizing radiation) but it is not currently well-established that the weaker radiation commonly emitted by electronic devices such as a CRT has long-term health effects (see Electromagnetic radiation hazard and Bio-electromagnetics).

Ionizing Radiation: CRTs also emit very small amounts of X-rays as a result of the electron beam's bombardment of the shadow mask/aperture grille and phosphors. Almost all of this radiation is blocked by the thick leaded glass in the screen so the amount of radiation escaping the front of the monitor is mostly harmless. The Food and Drug Administration regulations in 21 CFR 1020 are used to strictly limit, for instance, television receivers to 0.5 milliroentgens per hour (mR/h) (0.13 ┬ÁC/(kg·h) (at a distance of 5 cm from any external surface and as mentioned above, most CRT emissions fall well below this limit. Early color television receivers (many of which are now highly collectable, see CT-100) were especially vulnerable due to primitive high voltage regulation systems. X-ray production is generally negligible in black-and-white sets (due to low acceleration voltage and beam current) and virtually every color display since the late 1960s when systems were added to shut down the horizontal deflection system (and therefore high voltage supply) should regulation of the acceleration voltage fail.

All television receivers and CRT displays equipped with a vacuum tube based high voltage rectifier or high voltage regulator tube also generate X-rays in these stages, though these stages were universally housed in a metal enclosure called the "high voltage cage" to substantially reduce (and effectively eliminate) exposure. As examples, a 1B3 and a 6KB6 vacuum tube would be installed inside this metal enclosure. For both X-ray and electrical safety reasons, the set should never be operated with the cover of the high voltage cage opened. (Photo of HV cage to follow.)
 Toxins: Old CRTs may also have used toxic phosphors, although that is much less common today. An implosion or other breaking of the glass envelope could release these toxic phosphors. Because of the X-ray hazard, the glass envelopes of most modern CRTs are made from heavily leaded glass. The lead in this glass may represent an environmental hazard, especially in the presence of acid rain leaking through landfills. Indirectly-heated vacuum tubes (including CRTs) use Barium compounds and other reactive materials in the construction of the cathode and getter assemblies, normally this material will be converted into oxides upon exposure to the air, but care should be taken to avoid contact with the inside of all broken tubes. In some juristictions, all discarded CRTs are regarded as toxic waste.

Flicker: The constant refreshing of a CRT can cause headaches in migraine sufferers and seizures in epileptics, if they are photosensitive. Screen filters are available to reduce these effects. A high refresh rate (above 75 Hz) also helps to negate these effects.

High Voltage: CRTs operate at very high voltages. These voltages can persist long (several days) after the device containing the CRT has been switched off and unplugged. Residual charges of hundreds of volts can also remain in large capacitors in the power supply circuits of the device containing the CRT; these charges may persist for weeks. (Modern circuits contain bleeder resistors to ensure the high-voltage supply is discharged to safe levels within a couple of minutes at most.)

Those working inside CRT-containing equipment should know how and be able to safely discharge these hazards. In particular, the large rubber connector which looks like a suction cup is responsible for supplying accelerating voltage to the bell of the CRT. Under the suction cup is the ultor which couples the accelerating voltage to the inside of the tube. Inside the glass bell is a coating of metallic paint, while the outside of the bell is coated with a conductive graphite coating called Aquadag; between the ultor's connection to the flyback transformer and the Aquadag, there is therefore a capacitance capable of maintaining the full accelerating voltage for weeks. While this accelerating voltage is high (typically from 7kV to 50kV depending on screen size, monochrome or color, direct view or projection), both the capacitance and flyback current are small (on the order of picofarads and nanoamperes respectively), so shocks from the accelerating voltage are typically embarrassing and painful but usually harmless. On the other hand, the voltages and available currents used in the deflection and power supply circuits can result in instantaneous death.

Implosion: All CRTs and other vacuum tubes operate under high vacuum so that air and gas molecules will not interfere with electron streams. CRTs have large viewing areas and proportionally larger bells required to accommodate the deflection of the electron beams to the rear of the screen. As a result, these highly evacuated glass bulbs have a large surface area, with each and every square inch exposed to atmospheric pressure.

As an example, consider a 17-inch (16-inch viewable) CRT at a mean sea-level atmospheric pressure of 14.7 pounds per square inch. Measuring the visible portion of the CRT and rounding up to the nearest inch (accounting for invisible portions of the face), a Viewsonic model E771 monitor has a screen of 13x10 inches, or 130 square inches. At 14.7 PSI exterior pressure and a near-perfect internal vacuum, the face of this monitor is supporting over 1,900 pounds of air mass on its face alone. The entire CRT is conservatively supporting three times that - or nearly 6,000 pounds, the weight of three typical automobiles - across its entire surface. The larger the CRT, the more surface area, the more total exterior air pressure load.

Therefore, CRTs (outside of finished end-user products) present a hazard to those without proper training and appropriate precautions. While a great deal of research has gone into implosion protective designs for CRTs, all CRTs present an implosion risk. Even CRTs in finished products present a hazard if handled uncautiously. Early television receivers even included a "safety glass" to protect viewers from flying glass due to spontaneous structural failures of the CRT; with modern (early 1960s onward) banded and bonded-face CRTs, the safety glass has become redundant. Safety goggles, leather gloves, and heavy sweaters are considered indispensable safety equipment amongst experienced technicians and preservationists of early television equipment.

High Vacuum Safety
Because of the strong vacuum within a CRT, they store a large amount of mechanical energy; they can implode very forcefully if the outer glass envelope is damaged. Most modern CRTs used in televisions and computer displays include a bonded, multi-layer faceplate that prevents implosion if the faceplate is damaged, but the bell of the CRT (back portions of the glass envelope) offers no such protection. Certain specialized CRTs (such as those used in oscilloscopes) do not even offer a bonded faceplate; these CRTs require an external plastic faceplate or other cover to render them implosion safe while in use. Before the use of bonded faceplates one of the hazards would be that a broken neck or envelope would cause the neck and electron gun to be propelled by atmosperic pressure at such a velocity that it would erupt through the face of the tube.

Unmounted CRTs should always be carried with its face, the heaviest part, down. Use both hands, and grasp the tube under the face, wrapping your hands around to the sides where the metal mounting frame is attached. Never carry a CRT by the neck! For added safety, carrying the tube in a closed, thick box or with a thick cloth wrapped around it (but not in such a way as to impair your grip on the tube) is a good idea; this will reduce the amount of flying debris should the tube break. Large tubes (over 19 inches) should be carried by two people. In general, you should treat the tube like a hand grenade, thinking that if you handle it carefully and keep your grip on it, there is no serious danger, but that it could cause a disaster if you drop it.

When handling or disposing of a CRT, you must take steps to avoid creating an implosion hazard for yourself or your trash removal service. The most simple and safe method to make the tube safe is to identify the small sealed glass nib at the far back of the tube (this may be obscured by the electrical connector) and then (while wearing safety glasses and gloves) filing a small nick across this and then to break it off using a pair of pliers. A loud sucking sound will be heard as the air enters the tube, filling the vacuum. Once the vacuum is filled, the tube is destroyed, but it cannot implode. One must be very cautious not to break the neck of the tube when it is evacuated since there is no plastic coating preventing shattering of the glass. High vacuum and high voltage can be dangerous.


No comments:

Post a Comment